Need some advise on what would give me some more bang in the audio department. Choosing between M60 or an EP500

Setup at this time is
1 VP150 Centre channel
2 Custom fronts - ( 1-horntweeter/1-midrange/2- 15" subs) (speakers are 20 years old)
2 QS8 surrounds
2 M3Ti back speakers
1 Paradigm 8" powered sub
Powered by a Yamaha 5790 AV receiver.

The front stage seems to be balanced as I hear a seamless transition between speakers. But I keep wondering if I am missing something with 20 year old technology.
The Paradigm sub was bought to keep the whole home theatre on budget at the time, as well as thinking that the 4-15" subs in the front speakers could give me some bass.

What should I purchase first - new fronts or new sub?